The present invention pertains to an apparatus and method for encoding data for delivery over a multi-channel data delivery system and for decoding the received encoded date after delivery. Specifically, the present invention relates to an application of trellis coded modulation (TCM) to a multi-channel communication system.
The prior art of single-channel TCM involves a data transmission technique wherein the conventionally separate processes of forward error correction (FEC) coding and modulation are combined into one process. This joining of coding and modulation mapping allows maximization of the minimum Euclidean distance between all possible coded modulated symbol sequences, resulting in significant coding gains over uncoded multilevel modulation techniques without comprise of bandwidth efficiency or reduction of the effective information rate.
J. Massey introduced a considerable performance improvement resulting from this joining of coding and modulation processes in "Coding and Modulation in Digital Communications" (Proc. 1974 Int. Zurich Seminar Digital Commun., Zurich, Switzerland, March. 1974, pp. E2(1)-E2(4)). Later, in June 1976, G. Ungerboeck and I. Csajka introduced the first TCM schemes at the International Symposium on Information Theory in Ronneby, Sweden.
It was not until the early 1980's however, that significant applications and implementations of TCM schemes began emerging. The basic principles of TCM were published by G. Ungerboeck in "Channel Coding with Multilevel/Phase Signals" (IEEE Trans. Information Theory, Vol. IT-28, January 1982, pp. 55-66). Further key research was published by G. D. Forney et al. and L. F. Wei in "IEEE Trans. Selected Areas In Comm." (vol, SAC-2, September 1984).
Several applications and uses of TCM are disclosed in U.S. Patents issued to W. Lee-Fang, including U.S. Pat. No. 4,713,817 entitled "Multidimensional, Convolutionally Coded Communication Systems", U.S. Pat. No. 4,641,327 entitled "Frame Synchronization in Trellis-Coded Communication Systems", U.S. Pat. No. 4,520,490 entitled "Differentially Nonlinear Convolutional Channel Coding with Expanded Set of Signalling Alphabets", and U.S. Pat. No. 4,483,012 entitled "Differentially Convolutional Channel Coding with Expanded Set of Signalling Alphabets".
The prior art of single-channel TCM involves the use of combined forward error correction/modulation in a transmitter and corresponding demodulation/decoding in a receiver to reduce the number of data errors at the receiver. In general, TCM schemes employ an encoder to generate, based on the input data, a coded modulated symbol sequence from an expanded signal set. In the receiver, some of the received data errors are corrected by a decoder. The decoder selects the sequence most likely to have been transmitted by selecting the allowed sequence that most resembles (in term of squared Euclidean distance) the actual received, noisy sequence. The term "trellis" in trellis coded modulation was introduced because the allowed sequences in a particular TCM scheme can be described by a state transition (trellis) diagram similar to the trellis diagrams of prior art binary convolutional codes.
Use of trellis coded modulation over a given channel allows an increase in data throughput rate without degradation of the received data error rate, with respect to uncoded transmission. TCM can also be used to achieve improved performance at the same data throughput rate, with respect to uncoded transmission. In 1984, a TCM scheme was adopted by the International Telegraph and Telephone Consultative Committee (CCITT) for use in high-speed voiceband modems ("Recommendation V.32 for a family of 2-wire, duplex modems operating on the general switched telephone network and on leased telephone-type circuits," document AP VIII-43-E, May 1984). The resulting coding gains allow reliable transmission of data at higher rates over voiceband channels than were previously achieved with uncoded transmission. Single-channel TCM techniques are currently also being applied to other bandwidth-constrained channels, such a satellite, terrestrial microwave, and mobile communications.
In the prior art, when TCM is utilized for more than one channel of a multi-channel system, a separate encoder and decoder is allocated to each available channel, and the coding is independently applied along each channel. This technique is implementationally complex, since it requires N encoders and N decoders for an N-channel system. Equivalently, in the prior art, a single encoder and decoder hardware set could be multiplexed, where the set is used to encode and decode the data for each channel, and the encoder and decoder states for each channel are saved while the hardware is used to process the date for the other channels. This prior art application of TCM to an N-channel system is functionally equivalent to use of N encoders and N decoders, and is implemenationally complex because of the large quantities of state information that must be saved. The multi-channel trellis encoder/decoder of the present invention introduces an apparatus and method for application of TCM across channels in a multi-channel environment, resulting in reduced implementation complexity and also reduced data throughput delay with respect to the prior art, while retaining the ability to handle variations from channel to channel in data transmission rates, signal power, and noise power.